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[2003:cb:c733:f100:75e7:a0a4:9ac2:1abb]) by smtp.gmail.com with ESMTPSA id b10-20020a05600c4e0a00b00412e445a6e4sm2807340wmq.25.2024.03.04.06.27.50 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 04 Mar 2024 06:27:50 -0800 (PST) Message-ID: <17b4527c-3782-4eab-8b33-e0c6ff57139f@redhat.com> Date: Mon, 4 Mar 2024 15:27:49 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH] mm: hold PTL from the first PTE while reclaiming a large folio To: Ryan Roberts , Barry Song <21cnbao@gmail.com>, akpm@linux-foundation.org, linux-mm@kvack.org Cc: chrisl@kernel.org, yuzhao@google.com, hanchuanhua@oppo.com, linux-kernel@vger.kernel.org, willy@infradead.org, ying.huang@intel.com, xiang@kernel.org, mhocko@suse.com, shy828301@gmail.com, wangkefeng.wang@huawei.com, Barry Song , Hugh Dickins References: <20240304103757.235352-1-21cnbao@gmail.com> <706b7129-85f6-4470-9fd9-f955a8e6bd7c@arm.com> <37f1e6da-412b-4bb4-88b7-4c49f21f5fe9@redhat.com> <10f9542e-f3d8-42b0-9de4-9867cab997b9@arm.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 1F02A14001C X-Stat-Signature: 53j88pz4s84wb3urybj8hexhdqz3ne4g X-Rspam-User: X-HE-Tag: 1709562476-856497 X-HE-Meta: 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 mZVszji2 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 04.03.24 14:03, Ryan Roberts wrote: > On 04/03/2024 12:41, David Hildenbrand wrote: >> On 04.03.24 13:20, Ryan Roberts wrote: >>> Hi Barry, >>> >>> On 04/03/2024 10:37, Barry Song wrote: >>>> From: Barry Song >>>> >>>> page_vma_mapped_walk() within try_to_unmap_one() races with other >>>> PTEs modification such as break-before-make, while iterating PTEs >>>> of a large folio, it will only begin to acquire PTL after it gets >>>> a valid(present) PTE. break-before-make intermediately sets PTEs >>>> to pte_none. Thus, a large folio's PTEs might be partially skipped >>>> in try_to_unmap_one(). >>> >>> I just want to check my understanding here - I think the problem occurs for >>> PTE-mapped, PMD-sized folios as well as smaller-than-PMD-size large folios? Now >>> that I've had a look at the code and have a better understanding, I think that >>> must be the case? And therefore this problem exists independently of my work to >>> support swap-out of mTHP? (From your previous report I was under the impression >>> that it only affected mTHP). >>> >>> Its just that the problem is becoming more pronounced because with mTHP, >>> PTE-mapped large folios are much more common? >> >> That is my understanding. >> >>> >>>> For example, for an anon folio, after try_to_unmap_one(), we may >>>> have PTE0 present, while PTE1 ~ PTE(nr_pages - 1) are swap entries. >>>> So folio will be still mapped, the folio fails to be reclaimed. >>>> What’s even more worrying is, its PTEs are no longer in a unified >>>> state. This might lead to accident folio_split() afterwards. And >>>> since a part of PTEs are now swap entries, accessing them will >>>> incur page fault - do_swap_page. >>>> It creates both anxiety and more expense. While we can't avoid >>>> userspace's unmap to break up unified PTEs such as CONT-PTE for >>>> a large folio, we can indeed keep away from kernel's breaking up >>>> them due to its code design. >>>> This patch is holding PTL from PTE0, thus, the folio will either >>>> be entirely reclaimed or entirely kept. On the other hand, this >>>> approach doesn't increase PTL contention. Even w/o the patch, >>>> page_vma_mapped_walk() will always get PTL after it sometimes >>>> skips one or two PTEs because intermediate break-before-makes >>>> are short, according to test. Of course, even w/o this patch, >>>> the vast majority of try_to_unmap_one still can get PTL from >>>> PTE0. This patch makes the number 100%. >>>> The other option is that we can give up in try_to_unmap_one >>>> once we find PTE0 is not the first entry we get PTL, we call >>>> page_vma_mapped_walk_done() to end the iteration at this case. >>>> This will keep the unified PTEs while the folio isn't reclaimed. >>>> The result is quite similar with small folios with one PTE - >>>> either entirely reclaimed or entirely kept. >>>> Reclaiming large folios by holding PTL from PTE0 seems a better >>>> option comparing to giving up after detecting PTL begins from >>>> non-PTE0. >>>> >> >> I'm sure that wall of text can be formatted in a better way :) . Also, I think >> we can drop some of the details, >> >> If you need some inspiration, I can give it a shot. >> >>>> Cc: Hugh Dickins >>>> Signed-off-by: Barry Song >>> >>> Do we need a Fixes tag? >>> >> >> What would be the description of the problem we are fixing? >> >> 1) failing to unmap? >> >> That can happen with small folios as well IIUC. >> >> 2) Putting the large folio on the deferred split queue? >> >> That sounds more reasonable. > > Isn't the real problem today that we can end up writng a THP to the swap file > (so 2M more IO and space used) but we can't remove it from memory, so no actual > reclaim happens? Although I guess your (2) is really just another way of saying > that. The same could happen with small folios I believe? We might end up running into the folio_mapped() after the try_to_unmap(). Note that the actual I/O does not happen during add_to_swap(), but during the pageout() call when we find the folio to be dirty. So there would not actually be more I/O. Only swap space would be reserved, that would be used later when not running into the race. -- Cheers, David / dhildenb